Call it duck daycare. Some animals, particularly waterfowl, group different families of fledglings together for easier care. This is called a crèche, which is French for nursery.

A week ago, I was startled into writing this column by a group of 31 common mergansers. Just before the Long Falls Dam Road disappears into the woods behind Flagstaff Lake on the back side of Bigelow Mountain, there is a spot where the Dead River bends toward the pavement. There are mergansers here year-round since the river stays open in winter and the food fish never leave unless they are caught by anglers or eaten by ducks.

Most of the young mergansers were nearly adult-size and, at this age, even the males have red heads. It was tricky to figure out how many actual mommy ducks were in the flock. Fortunately, the young had grayish bills and the hens had reddish bills, so I’d estimate three adults and 28 kids. This type of crèche behavior is unusual but not rare. It is most typical among birds that breed colonially and whose eggs hatch at roughly the same time. Generally, it occurs more often when the chicks are able to feed themselves after leaving the nest.

Some species of tern gather their young together. Flamingos and ostriches do it. You’ve probably seen it among Canada geese and common eiders. Eider crèches have contained up to 150 babies. King penguins will herd thousands of penguinettes into a huddle to conserve heat.

Common, red-breasted, and hooded mergansers form crèches in Maine. The advantages are numerous. For mama duck, it allows other mamas to take turns watching over the kids while she feeds. Daddy merganser has nothing to do with raising the kids and is notably absent from these crèches, so mom can use all the help she can get. Often, aunts pitch in to help mind the crèche.

There is much that scientists need to puzzle out. On the surface, there is little evolutionary benefit to mama duck when she helps raise another hen’s chicks. Evolution favors actions that pass along the mother’s genes, not the neighbor’s. The duck that volunteers to tend unrelated youngsters exposes herself to danger and fatigue, with no apparent advantage to her own offspring. There is a theory in nature called the dilution principle. It holds that individual birds in large flocks are less likely to be picked off by a predator because the marauder has so many other alternative targets. Perhaps a mom who surrounds her own chicks with other chicks creates safety in numbers for her brood, especially if she can keep hers in the middle.

Bunching the babies doesn’t always work. Common eider chicks in Maine are being decimated, some by eagles but most by gulls, especially black-backed gulls. Even in a flock protected by multiple hens, persistent gulls are able to pick off the youngsters one by one. Fewer eider chicks are surviving to maturity. In some locations, up to 90 percent of chicks become gull food. In the late 1800s, eiders were nearly hunted to extinction. When conservation efforts started and the population rebounded, there were few herring gulls in Maine and no black-backed gulls to impede progress.

Other animals form crèches. Lions form prides. Speckled caimans, a crocodilian reptile, gather their babies together. Some schools of fish demonstrate crèche behavior. Bats do it.

My 31 mergansers confirmed that individual competition within a crèche is vigorous. As we watched, one lucky duck clamped onto a trophy trout — a whale of a trout. Fish-eating birds, if given a chance, can swallow prey much larger than you’d expect. None of the other mergansers were willing to offer that chance. As it tried to flip the fish in order to swallow it head first, 30 other mergansers gave hot pursuit, trying to snatch it away. They raced over water and — much to my surprise — over land, a stream of running, clambering ducks. They scrambled up rocks, down the beach, over the river, up the bank, and over the rocks again.

This isn’t easy for diving ducks whose feet are placed too far back on the body for easy walking. Despite the odds, lucky ducky managed to shoulder off the competition and…gulp.